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Sender: Tejun Heo <htejun@gmail.com>
Date:   Tue, 26 Jul 2022 13:30:40 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        John Stultz <john.stultz@linaro.org>,
        Dmitry Shmidt <dimitrysh@google.com>,
        Oleg Nesterov <oleg@redhat.com>, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org
Subject: Re: [PATCH 2/3 cgroup/for-5.20] cgroup: Add "no" prefixed mount
 options
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Hello,

On Tue, Jul 26, 2022 at 10:01:04AM -1000, Tejun Heo wrote:
> On Tue, Jul 26, 2022 at 04:32:46PM +0200, Michal Koutn? wrote:
> > On Thu, Jul 14, 2022 at 06:38:43PM -1000, Tejun Heo <tj@kernel.org> wrote:
> > > We allow modifying these mount options via remount. Let's add "no" prefixed
> > > variants so that they can be turned off too.
> > 
> > They can be turned off:
> > 
> > > // on v5.19-rc?
> > > :~ # grep cg /proc/mounts
> > > cgroup2 /sys/fs/cgroup cgroup2 rw,nosuid,nodev,noexec,relatime,nsdelegate 0 0
> > > :~ # mount -t cgroup2 cgroup2 /sys/fs/cgroup/ -oremount
> > > :~ # grep cg /proc/mounts
> > > cgroup2 /sys/fs/cgroup cgroup2 rw,relatime 0 0

  root@test ~# grep cgroup /proc/mounts
  cgroup2 /sys/fs/cgroup cgroup2 rw,nosuid,nodev,noexec,relatime,nsdelegate,memory_recursiveprot 0 0
  root@test ~# mount -o remount,exec /sys/fs/cgroup
  root@test ~# grep cgroup /proc/mounts
  cgroup2 /sys/fs/cgroup cgroup2 rw,nosuid,nodev,relatime,nsdelegate,memory_recursiveprot 0 0
  root@test ~# mount -o remount /sys/fs/cgroup
  root@test ~# grep cgroup /proc/mounts
  cgroup2 /sys/fs/cgroup cgroup2 rw,nosuid,nodev,relatime,nsdelegate,memory_recursiveprot 0 0
  root@test ~# mount -o remount,nsdelegate,memory_recursiveprot cgroup2 /sys/fs/cgroup
  root@test ~# grep cgroup /proc/mounts
  cgroup2 /sys/fs/cgroup cgroup2 rw,relatime,nsdelegate,memory_recursiveprot 0 0
  root@test ~# mount -o remount,memory_recursiveprot cgroup2 /sys/fs/cgroup
  root@test ~# grep cgroup /proc/mounts
  cgroup2 /sys/fs/cgroup cgroup2 rw,relatime,memory_recursiveprot 0 0
  root@test ~# mount -o remount cgroup2 /sys/fs/cgroup
  root@test ~# grep cgroup /proc/mounts
  cgroup2 /sys/fs/cgroup cgroup2 rw,relatime 0 0

Man, I had no idea that `mount -o remount,$OPTS $MOUNT_POINT` and `mount -o
remount,$OPTS $SRC $MOUNT_POINT` behave completely differently in how they
handle existing options. I wonder why other filesystems are implementing
explicit no prefixed options.

Anyways, will soon post a patch to remove the no prefixed options.

Thanks for pointing it out.

-- 
tejun